Glass scratches too though. Its pretty simple...things will scratch when it comes in contact with things that are harder than it. Check out the Mohr scale. Glass (quartz) is somewhere around 6. High quality watches will have sapphire crystal (which has a hardness around 9). And diamond is 10.

So unless manufacturers start making sapphire crystal faces on their phones (which would be quite heavy), scratching will always be an issue...especially with glossy covers/surfaces.